Imaging systems of both planar and internal drum design are known in the art. These devices are used in the graphics arts fields as well as in the fabrication of printed circuit boards. Planar imaging systems, such as are disclosed and claimed in U.S. Pat. No. 4,851,656 and incorporated herein by reference, are types of imaging systems which have a planar surface for receiving a substrate. An optical exposure head is located on a movable gantry apparatus and is rastered above the substrate during exposure. Internal drum devices, on the other hand, have a cylindrical drum surface portion to receive a substrate. An optical beam generator emits an optical beam onto a spinning mirror, and the mirror reflects the beam onto the substrate. As the mirror spins, the reflected beam advances across the substrate surface from a starting edge of the surface to an ending edge thereof, exposing a sequence of pixels which together form a scan line perpendicular to the axis of the drum. The spinning mirror is mounted on a carriage which moves in a direction perpendicular to the scan line. After the reflected beam advances to the ending edge, the carriage moves perpendicular to the scan line. When the reflected beam begins again at the starting edge of the surface, the beam advances across a new scan line. The reflected beam advances across the entire surface area of the drum in this manner.
A laser diode is a known device for generating optical beams and comprises a p-n semiconductor junction which generates coherent, substantially single-wavelength, highly directional electromagnetic radiation when drive current into the diode reaches a lasing threshold level. A description of laser diodes can be found in "Viable Laser Diodes" and "Semiconductor Laser Diodes", both of which are in the 1996 Photonics Design and Applications Handbook, incorporated herein by reference.
Laser diodes are commercially available in a variety of power output levels and optical radiation wavelengths. The output power of the laser diode optical beam depends on the electrical current provided to drive the laser diode low values of drive current, the output power is low. The laser diode thereby operates as a light emitting diode (LED) in a first operational mode. In the LED mode, the diode emits an incoherent optical beam in a broad emission "cone" extending from the edge of the diode. The LED beam is comprised of radiation of many different wavelengths and many optical modes. Beyond a value of drive current known as the lasing threshold, the laser diode operates in a second operational mode known as lasing mode. In the lasing mode, the diode emits a laser beam of a single narrow wavelength band and optical quality which approaches single longitudinal mode. As discussed hereinbelow, the laser beam has several characteristics rendering it ill-suited for use in an imaging system.
To use the laser diode in an imaging system, the system must present a modulated laser beam of controlled power to image a substrate. One method of accomplishing the same is to switch the diode on and off between controlled states. A rate at which the laser diode can switch between an "on" state, wherein the diode emits a laser beam, and an "off" state, wherein the diode does not emit a laser beam, determines an imaging rate. In the "off" state, drive current is typically zero, while in the "on" state drive current is typically a current level which is above the lasing threshold. Thus, a difference between beam power when in the "on" state and beam power when in the "off" state is exactly the beam power of the laser beam when in the "on" state. A ratio of power of the beam in the "on" state to the power of the beam in the "off" state defines a contrast ratio. As is known in the art, a high contrast ratio is preferred in order to create a sharp image on the substrate. In an imaging system where the diode emits no beam in the "off" state, the contrast ratio is infinite. A large contrast ratio is highly desirable because it permits a full range of exposure control to service the demands of different media, exposure rates, i.e. scan rates, and resolution.
Unfortunately, the time required to switch from the one current to the off current limits the imaging rate. Though narrowing the difference between the off current and the on current increases the imaging rate, it increases the beam power when in the "off" state and thereby lowers the contrast ratio. Another option for intermittently emitting a laser beam to image a substrate is to keep the diode in the lasing mode and vary the continuously emitted laser beam with a modulator which is external to the diode. Types of modulators include acousto-optic modulators known as Bragg cells as well as electro-optic modulators known as Pockels cells and Kerr cells. However, these external modulators are costly and must be precisely aligned with the laser beam. Aligning optical components and maintaining their alignment is a time consuming and error prone task.
To use a beam generator in an imaging system, the beam generator must emit a collimated laser beam which has a desired diameter, wavelength and power level and which is circularly symmetrical and diffraction limited. A laser beam which is circular symmetrical and diffraction limited forms an imaging spot upon the substrate which is well defined and has a maximal focus depth, and requires a minimal beam diameter. Unfortunately, a laser beam produced by a laser diode has an anamorphic divergence pattern, rather than a symmetrical divergence pattern, and also suffers from astigmatism. These deficiencies in the laser beam are departures from true single mode behavior. Conventional imaging systems employ precisely aligned and costly multi-element lenses and prisms to alter a laser beam so that it is circular symmetrical and diffraction limited. However, even these costly remedies do not improve the limitations in contrast ratio and imaging rate.
In addition, in conventional optical beam generators in which a laser diode generates the optical beam, the diode is located within a housing mounted to the imaging system. If the laser diode fails, the module which houses the diode must be removed from the imaging system, and the diode is removed from the housing and replaced. The housing is then remounted on the imaging system and precisely realigned with other components of the imaging system such as lenses and mirrors. It would be advantageous to provide an optical beam generator which does not require realignment after a failed laser diode is replaced.
It would be advantageous to provide an optical beam generator which emits a collimated optical beam having a desired diameter, wavelength and power level, and which optical beam is furthermore circularly symmetrical and diffraction limited.
It would be further advantageous to provide an optical beam generator with a high imaging rate and which emits an optical beam having a high contrast ratio. The present invention is directed towards such a system.